Since the mid-90s, the principle of functional equivalence has been used as a guideline for the regulation of the information society. Its aim is to give legal value to the use of electronic means during the contractual process. According to this principle, electronic documents or data should be considered as equivalent to paper documents if they fulfil the same functions. For instance, considering the legal requirement of a signature, the idea is to take some distance from paperbased environment and handwritten graphics in order to identify the fundamental functions the signature is supposed to fulfill. In this respect, scholars usually consider that a signature is used as a means to authenticate a person, i.e. to identify her and to give consent. Therefore, any electronic means that ensures the authentication function should be equivalent to a handwritten signature. This reasoning also involves principles such as technology neutrality and non-discrimination. The principle of functional equivalence was adopted in 1996 by the United Nations Commission on International Trade Law (UNCITRAL) in its model law on electronic commerce. Since then, it has been recognized by many lawmakers around the world. Nevertheless, a certain lack of methodology has resulted in diverging initiatives and implementations. In this context, the aim of this study is to build a theory of functional equivalence based on a comparative and interdisciplinary approach. First it identifies the roots, evolution, forces and weaknesses of the principle. Then it develops its conceptual structure and methodology. For a better understanding of the principle, the first part of the study proposes a cartography of the genesis and propagation of the principle, both at the international and national level. It goes back to the Scandinavian origins of the principle and underlines the influence exerted by methods of Value Analysis. The second part of the study proposes a critical and theoretical model based on the principles of equivalence and technology neutrality. A functional methodology taking into account the teachings of philosophy of science and value analysis reasoning is developed with the hope it will serve as a guide for the lawmaker. Particular attention is devoted to the core notion of function, the selection process of the relevant functions of a given formal requirement and the best way to express their meaning and scope. Finally, the proposed method is tested on three formal requirements: writing, information and signature.